There have been proposed several alternative processes normally grouped under the title of "smelting reduction" processes, in which the use of carbonaceous material through addition or injection in a vessel containing molten metal aims at primary objectives departing from obtaining liquid steel within the same reactor. Such is the case of processes where the vessel can be considered as a coal gasification unit, and thus the metal produced is of secondary importance, or else in processes grouped under the title of "direct steelmaking" processes, where the gases generated are further oxidized, i.e., postcombusted within the same vessel aiming at the achievement of extra energy. However, this last aspect is widely recognized to be a low energy efficient process since heat that could be transferred to the metal is well below 50% in reference to that generated by postcombustion of gases. This situation also limits further use of the gases for direct reduction process application or as a fuel combustion gas in other processes.
The above mentioned processes require preferably the use of carbonaceous materials with high fixed carbon and low ash contents in order to increase the amount of solid metallic charge to be melted in the vessel. These carbonaceous materials in most of the cases are known to be in decreased availability, such as in the case of antracite or as in the case of by products such as coke, breeze or lignite coke which additionally have the disadvantage of being more expensive.
The processes that use the generated gases for direct reduction purposes, i.e., no postcombustion is practiced, also grouped as "direct steelmaking" processes, are said to operate in a non-repeated two basic period type of practice, the main period being the simultaneous injection of carbonaceous material and oxygen and the addition of solid metallic charge, e.g., sponge iron, during a relatively prolonged period of time in an attempt to achieve both nearly constant melt carbon content and temperature at stationary conditions, as measured by decarburization rate of melt being (dc/dt) around zero, until a given volume of molten metal is completed. The final period is relatively short in time and performed under oxidizing conditions for refining of the metal to a degree where steel can be tapped, or a metal for further treatment is obtained. There clearly exists an inherent difficulty to control process variables and desired conditions when the mass is incremented with time, such variables including rates of injection and additions of solid metallic charge, carbonaceous material, oxygen, fluxes for slagging impurities, and such conditions, including composition of the melt as measured by carbon content and melt temperature.
According to the inventor's experience, it has been found that it is not possible in practice to exert an effective control of the process to achieve such mentioned stationary or steady state condition while the mass is being increased. Furthermore, the inventor's work has proved that by using multiple alternating and separated reducing and oxidizing condition periods, including the addition of solid charge during the initial stage of the oxidizing period, it is possible to overcome the control difficulties mentioned and achieve the performance required by the process.